The invention relates to a method of processing the digital sound/data components of a time multiplexed television signal, discrete lines of which sequentially contain a digital sound/data component at a first bit rate and a vision component. The invention also relates to apparatus using the above method and a television transmission system and an associated conversion unit and a receiver which use the above method.
Following the decision in March 1982 that direct broadcast by satellite (DBS) of television programs for the United Kingdom would commence in 1982, an Advisory Panel chaired by Sir Anthony Part was established to report on technical transmission standards. The findings of this Panel, published in November, 1982 by Her Majesty's Stationery Office as Cmnd 8751 "Direct Broadcasting by Satellite --Report of the Advisory panel on Technical Transmission Standards" (known as the Part Report), recommended that the Independent Broadcast Authority's Multiplexed Analogue Component (C-MAC) system should be adopted for DBS, which recommendation has subsequently been accepted.
The C-MAC system has been described in the Independent Broadcast Authority's Experimental and Development Report 118/82 "MAC--A Television System for High-Quality Satellite Broadcasting" dated August, 1982, this report also describing the A-MAC system (the prefix relating to the type of sound and data transmission). Proposed specifications were included in this report for the two systems, that for the C-MAC system having been revised since the adoption of that system for DBS. The changes in the structure of the video waveform have been a reduction in transition periods following the sound/data, chrominance and luminance (vision) components with a consequent lengthening of the sound/data component
FIG. 1 of the accompanying drawings (which is not to scale) diagrammatically shows one line period of a C-MAC television signal, derived from a draft specification for the system, which occupies 64 .mu.s and each line is divided nominally into a number of bit or sample periods at a clock rate of 20.25 MHz, there being 1296 such samples per line. Each line contains the following in the sequence given:
a=194 bits--synchronization, sound/data PA1 b=4 samples--transition from end of data PA1 c=15 samples--main clamp period (zero-level of chrominance reference) PA1 d=355 samples--chrominance (C) PA1 e=4 samples--grey-to-black transition PA1 f=10 samples--black level clamp period (block level reference PA1 g=710 samples--luminance (Y) PA1 h=4 samples--transition into data. PA1 a=203 bits--synchronization, sound/data (data burst). PA1 b=4 samples--transition from end of data. PA1 c=15 samples--main clamp period (zero-level of chrominance reference). PA1 SC1=6 samples--reserved for vision scrambling. PA1 d=354 samples--chrominance (C). PA1 g=704 samples--luminance (Y). PA1 SC2=6 samples--reserved for vision scrambling. PA1 h=4 samples--transition into data.
The chrominance component is time compressed at a rate of 3:1 so that 52.59 .mu.s of chrominance information is compressed to occupy 17.53 .mu.s (355 samples) with the R-Y color difference signal being transmitted on alternate lines and the B-Y color difference signal being transmitted on the intervening lines. The luminance component is time compressed at a rate of 3:2 so that 52.59 .mu.s of luminance information is compressed to occupy 35.06 .mu.s (710 samples). For DBS transmissions, the compressed chrominance and luminance components are frequency modulated with a bandwith of 27 MHz while the radio frequency carrier is modulated using 2-4 phase shift keying (2-4 PSK) by the digital sound/data component. The exact nature of the sound/data component has not yet been decided but examples are given in the above mentioned documents.
The above specification has been further revised in the European Broadcasting Union Draft New Report "Television Standards for 625-line 12 GHz Satellite Broadcasting", SPB 284, dated June, 1983, and each line contains the following in the sequence given;
From this EBU Draft Standard it will be seen that the components e and f have been omitted while component SC1 has been inserted between components c and d and component SC2 has been inserted between components g and h. In addition, the chrominance component amplitude has been changed to make it the same as the luminance component. However, these changes are not important for an understanding of the invention.
While it will be possible for housholds to directly receive the DBS transmission by means of a dish aerial of appropriate size sighted onto the satellite with a down-converter at the aerial to bring the frequency of the incoming transmission to just above the broadcast U.H.F. bands, it has also been suggested that many households will prefer to receive such transmission via a cable television distribution system which, at the same time, can convey other television programs while doing away with the need for individual aerials. Such distribution by way of cable will obviously have advantages where the signal from the satellite is weak, e.g. the transmission is not primarily intended for the country in which it is received, and where transmissions are being received from a number of satellites located in different geostationary positions thus requiring a complex aerial array.
Chapter 7 of the Part Report deals with the interaction between DBS and cable distribution systems and it is reported that the Cable Television Association of Great Britain believe that they would be able to provide a cable service even if C-MAC was chosen as the DBS transmission standard. Several examples are given in that chapter and, where appropriate to C-MAC, the inference is that this type of signal could be directly transmitted over cable systems. Present cable transmission systems use co-axial cable conveying their television programs in the V.H.F. broadcast bands and while there is much debate at present as to whether future installed systems should use optical fiber cables, it is quite likely that many of the systems yet to be installed will also be co-axial cable in view of lower installation cost.
It has recently been realized that the transmission of a C-MAC signal over a VHF cable transmission system is not as feasible as originally throught as the 27 MHz bandwidth of such a signal would occupy too much bandwidth thus reducing the number of programs that such a cable system could carry. In addition transmission of the sound/data component at the high 20.25 Mbit/s rate would pose severe problems on such cable systems because of the short delay reflections produced and that there is a much lower bit rate limit for such cable systems. With the above in mind, some sources have suggested that the only practical way of handling such a signal over a VHF cable transmission system is to convert the C-MAC signal into a PAl type signal prior to its application to a cable system. Such a conversion would lose the advantage of time multiplexed chrominance and luminance and re-introduce the defects of cross-luminance and cross-color present with color subcarrier systems, but, more important where the received DBS signal is scrambled to prevent unauthorized reception e.g. subscription television services, it would be necessary to descramble the signal prior to conversion and then rescramble the converted signal.
In addition, it has also been realized that the high bit rate of the digital sound/data component (20.25M bits/s) would make it difficult to process even in television receivers which will receive their transmissions directly from a satellite.